Computer Graphics

Computer-generated graphics are widely used in publishing, from simple

graphic elements for magazines and logos for corporate identity work to

complex technical illustrations and exploded diagrams for instruction

manuals. However, problems arise when publishers and/or printers do not

understand the two basic categories of computer graphics and the differences

between them.

There are two fundamentally different types of computer graphic files:

❑ raster,

❑ vector.

Raster graphics

Raster files describe the colour (and, therefore, tone) of every addressable

position of a given image – the pixel. Complex pictures are built up by varying

the attributes of the pixels within the image. The total image is stored as

a reference grid, or map, of each pixel’s value. In an image which is just

black and white, each pixel is either ‘on’ (black) or ‘off’ (white). Each pixel

only needs a single bit to record its state, hence the term ‘bitmapped image’.

In a tone or coloured image each pixel requires more than a single bit. These

pixels are said to have ‘depth’. The size of raster files corresponds directly

to the number of addressable positions and colours in the image.

Vector graphics

Vector files, on the other hand, contain no information about specific dots.

Instead, vector files are a list of mathematical descriptions of the boundaries

of graphic objects used to create an image. They are the building components

of the image rather than a representation of the finished graphic. In its simplest

form, a line, a vector file contains the data as to the line’s start position,

its length, the direction it travels and its weight and colour. To become an

image, the vector must be drawn by an application. Because the image is

redrawn, from fundamental instructions within the file, the resolution of the

final result is dependent on the output, or viewing device. Vector images are

not linked to any particular piece of hardware; it is up to the application software

to interpret the instructions correctly. It is the resolution-independence

of vector graphics which allows the same fonts to be written to a 300 dpi

office printer and a 3400 dpi imagesetter and be reproduced at the optimum

quality from both.